The plating of a metal surface with a metal film is widely employed for protecting it and improving its appearance. There is also proposed a composite plating material containing a metal and an ultrafine carbonaceous material having a diameter of 1.0 to 50 nm (nanometers) and called nanocarbon. The carbonaceous material is outstanding in thermal and electrical conductivity and can be combined with a metal to improve its thermal and electrical properties.
We, the inventors of this invention, have already proposed a composite plating material containing nanocarbon and a metal in Japanese Patent Application No. 2005-290463.
We have proposed a product coated with a composite plating layer formed by employing a composite plating solution prepared by adding carbon nanofibers and a surface active agent to a metallic plating solution. The composite plating layer contains 50 to 98% by volume of plating metal and 2 to 50% by volume of carbon nanofibers. Although carbon nanofibers are normally low in wettability, the surface active agent improves their wettability and thereby makes a product of good durability.
The product will be described with reference to FIG. 3.
FIG. 3 shows a section of a plating film 101 formed on an iron plate 100 by a combination of zinc deposited thereon and carbon nanofibers dispersed therein as examined under an electron microscope. The film 101 first has a thickness of less than 10 μm and contains carbon nanofibers dispersed therein substantially uniformly. Then, an uneven surface is formed thereon mainly by protruding zinc particles 102 containing carbon nanofibers, zinc deposited thereafter is concentrated on the protruding zinc particles 102 and carbon nanofibers 103 are also concentrated thereon and combined with zinc.
According to the basic principle of the growth of a plating film, the deposition of a metal ion does not occur uniformly on the whole surface of a plating film formed, but is concentrated on protruding portions thereof. As the deposition of the metal is concentrated on protruding portions, hardly any deposition or any appreciable growth of the plating film occurs in recessed portions. Accordingly, an uneven surface formed during the initial period of metal deposition becomes still more uneven with the progress of a plating operation.
The foregoing is considered to explain the reason why carbon nanofibers are concentrated on the protruding portions on which metal deposition is concentrated. In fact, the examination by an electron micrograph reveals the concentration of carbon nanofibers on protruding portions.
As the protruding portions are formed on its outer surface, the plating film 101 formed by a known method of composite plating does not improve the outward appearance of the iron plate 100. When a composite plating film, such as 101, formed on a metallic material, such as iron plate 100, to improve its thermal and electrical properties impairs its appearance, composite plating has only a limited scope of applicability. Composite plating is required to be able to form a smooth surface in order to have an expanded scope of applicability.